1. Field of the Invention
This invention generally relates to the fabrication of microelectromechanical systems (MEMS) and, more particularly, to a cantilever thin-film transistor (TFT) MEMS and related fabrication procedures.
2. Description of the Related Art
TFTs are formed through deposition processes that create thin films of silicon (Si) and insulator material. While the resulting TFTs may not have the switching speed and drive capability of transistors formed on single-crystal substrates, the transistors can be fabricated cheaply with a relatively few number of process steps. Further, thin-film deposition processes permit TFT devices to be formed on alternate substrate materials, such as transparent glass substrates, for use in liquid crystal displays (LCDs). More specifically, the TFTs include a deposited amorphous Si (a-Si) layer. To improve the performance of the TFT, the a-Si may be crystallized to form poly-silicon, at the cost of some extra processing. The crystallization procedures are also limited by the temperature sensitivity of the substrate material. For example, glass substrates are known to degrade at temperatures over 650 degrees C. Large scaled devices, integrated circuits, and panel displays are conventionally made using thin-film deposition processes.
MEMS devices are a logical derivative of semiconductor IC processes that may be used to develop micrometer scale structural devices such as transducers or actuators. MEMS devices interface physical variables and electronic signal circuits. MEMS structures are varied and, therefore, more difficult to standardize, as compared to the above-mentioned thin film processes. On the other hand, it may be possible to develop MEMS devices by engineering modifications to well-developed silicon IC processes. Many of the MEMS devices that have been fabricated to date have more theoretical than practical application, as the devices are often difficult and expensive to make. For the same reason, larger scale systems using MEMS components, have been expensive to fabricate due to the process difficulties and the cost associated with integrating the MEMS and IC technologies. For example, transistors and associated MEMS structures have been fabricated on bulk Si substrates, and the authors claim excellent performing bio-chemical sensing MEMS transducers [Vinayak P Dravid and Gajendra S Shekhawat; “MOSFET Integrated Microcantilevers for Novel Electronic Detection of “On-Chip” Molecular Interactions”, Material Science, Northwestern University, Evanston, Ill.]. However, the etching processes needed to form a bulk silicon MEMS are more difficult to control, dramatically limit available process steps, and require long etch times. These limitations make these devices unsuitable for low-cost integrated systems. Alternately, MEMS structures made using high temperature LPCVD thin films have been built with conventional sensing schemes such as capacitive and/or piezoresistive bridges, generating reasonable output signals [(1) William P. Eaton, James H. Smith, David J. Monk, Gary O'Brien, and Todd F. Miller, “Comparison of Bulk- and Surface-Micromachined Pressure Sensors”, Micromachined Devices and Components, Proc. SPIE, Vol. 3514, P. 431. (2) Joao Gaspar, Haohua Li, Paulo Peieiro Freitas, “Integrated Magnetic Sensing of Electro-statically Actuated Thin-Film Microbridges”, Journal of MicroElectroMechanical Systems, Vol. 12, No. 5, October 2003, p.550-556]. However, these sensing schemes cannot be applied to low temperature TFT process, because the changes in electrical characteristics induced as a result of stress change are too small to be practically measured.
It would be advantageous if active devices could be formed in a MEMS mechanical structure using the same, shared process steps.
It would be advantageous if a TFT could be integrated with a MEMS mechanical structure using the same, shared thin-film deposition and annealing processes.
It would be advantageous if a stress change sensing scheme could be formed in a MEMS mechanical structure using the same, shared process steps as TFT fabrication.